A ‘ringbuffer’ application Introduction to process ‘blocking’ and the Linux kernel’s support for ‘sleeping’ and ‘waking’

Devices might be ‘idle’ With our previous device-driver examples (i.e., dram, cmosram), the data to be read was already there, just waiting to be input But with certain other character devices, such as a keyboard, a program may want to input its data before any new data has actually been entered by a user In such cases we prefer to wait until data arrives rather than to abandon reading

Devices might be ‘busy’ Sometimes an application wants to ‘write’ some data to a character device, such as a printer, but the device temporarily is not able to accept more data, being still busy with processing previously written data Again, in such situations we prefer to just wait until the device becomes ready for us to send it more data rather than to give up

We could do ‘busy waiting’… It is possible for a device-driver to ‘poll’ a status-bit continuously until data is ready, (or until a device is no longer “too busy”): Such a technique is called ‘busy waiting’ But it could waste a lot of valuable CPU time before any benefit was realized! do { status = inb( 0x64 ); } while ( ( status & READY ) == 0 );

Avoid ‘busy waiting’ In a multitasking system we would want to avoid having any processes use the ‘busy waiting’ strategy whenever possible, as it ‘stalls’ any progress by other tasks – it’s a system-performance ‘bottleneck’! So modern operating systems support an alternative strategy, which allows those tasks that could proceed to do so

‘blocking’ while idle If a task is trying to read from a device-file when no data is present, but new data is expected to arrive, the operating system can ‘block’ that task from consuming any valuable CPU time while it is waiting, by ‘putting the task to sleep’ – yet arranging for that task to be ‘awakened’ as soon as some fresh data has actually arrived

‘blocking’ while busy Similarly, if a task is trying to ‘write’ to a device-file, but that device is ‘busy’ with previously written data, then the OS can put this task to sleep, preventing it from wasting any CPU time during its delay so that other tasks can do useful work – but arranging for this ‘sleeping’ task to be ‘woken up’ as soon as the device is no longer ‘busy’ and can accept fresh data

What does ‘sleep’ mean? The Linux kernel puts a task to sleep by simply modifying the value of its ‘state’ variable: –TASK_RUNNING –TASK_STOPPED –TASK_UNINTERRUPTIBLE –TASK_INTERRUPTIBLE … Only tasks with ‘state == TASK_RUNNING’ are granted time on the CPU by the ‘scheduler’

What does ‘wakeup’ mean? A sleeping task is one whose ‘task.state’ is equal to ‘TASK_INTERRUPTIBLE’ or to ‘TASK_UNINTERRUPTIBLE’ A sleeping task is ‘woken up’ by changing its ‘task,state’ to be ‘TASK_RUNNING’ When the Linux scheduler sees that a task is in the ‘TASK_RUNNING’ state, it grants that task some CPU time for execution

‘run’ queues and ‘wait’ queues In order for Linux to efficiently manage the scheduling of its various ‘tasks’, separate queues are maintained for ‘running’ tasks and for tasks that temporarily are ‘blocked’ while waiting for a particular event to occur (such as the arrival of new data from the keyboard, or the exhaustion of prior data sent to the printer)

Some tasks are ‘ready-to-run’ Those tasks that are ready-to-run comprise a sub-list of all the tasks, and they are arranged on a queue known as the ‘run-queue’ Those tasks that are blocked while awaiting a specific event to occur are put on alternative sub-lists, called ‘wait queues’, associated with the particular event(s) that will allow a blocked task to be unblocked

Kernel waitqueues waitqueue

Kernel’s support-routines The Linux kernel makes it easy for drivers to perform the ‘sleep’ and ‘wakeup’ actions while avoiding potential ‘race conditions’ that are inherent in a ‘preemptive’ kernel Your driver can use the support-routines by including the header:

Use of Linux wait-queues #include wait_queue_head_tmy_queue; init_waitqueue_head( &my_queue ); sleep_on( &my_queue ); wake_up( &my_queue ); But can’t unload driver if task stays asleep!

‘interruptible’ is preferred #include wait_queue_head_twq; init_waitqueue_head( &wq ); wait_event_interruptible( wq, ); wake_up_interruptible( &wq ); An ‘interruptible’ sleep can be awoken by a signal, in case you might want to ‘unload’ your driver!

A convenient ‘macro’ DECLARE_WAIT_QUEUE_HEAD( wq ); This statement can be placed outside your module’s functions (i.e., a ‘global’ object) It combines declaration with initialization: wait_queue_head_t wq; init_waitqueue_head( &wq );

Our ‘stash’ device Device works like a public ‘clipboard’ It uses kernel memory to store its data It allows ‘communication’ between tasks What one task writes, another can read!

Ringbuffer A first-in first-out data-structure (FIFO) Uses a storage array of finite length Uses two array-indices: ‘head’ and ‘tail’ Data is added at the current ‘tail’ position Data is removed from the ‘head’ position

‘ringbuffer’ depicted DATADATA HEAD TAIL DATADATA DATADATA DATADATA DATADATA FIFO rules: The next data to be added goes in at the current ‘tail’ position, and the next data to be removed comes from the ‘head’ position The ringbuffer is ‘empty’ when ‘head’ equals ‘tail’, and it is ‘full’ if ‘tail’ + 1 equals ‘head’ (modulo RINGSIZE)

Ringbuffer (continued) One array-position is always left unused Condition ‘head == tail’ means “empty” Condition tail == head-1 means “full” Both ‘head’ and ‘tail’ will “wraparound” Calculation: next = ( next+1 )%RINGSIZE;

read-algorithm for ‘stash’ if ( ringbuffer_is_empty ) { // sleep, until another task supplies some data // or else exit if a signal is received by this task } Remove a byte from the ringbuffer; Copy the byte to user-space; Awaken any sleeping writers; return 1;

write-algorithm for ‘stash’ if ( ringbuffer_is_full ) { // sleep, until some data is removed by another task // or else exit if a signal is received by this task } Copy a byte from user-space; Insert this byte into ringbuffer; Awaken any sleeping readers; return 1;

Demonstration of ‘stash’ Quick demo: we can use I/O redirection For demonstrating ‘write’ to /dev/stash: $ echo “Hello” > /dev/stash For demonstrating ‘read’ from /dev/stash: $ cat /dev/stash

The ‘device’ file-node We cannot use the ‘stash.c’ device-driver until a device-node has been created that allows both ‘read’ and ‘write’ access (the SysAdmin must usually do this setup): #root mknod /dev/stash c 40 0 #root chmod a+rw /dev/stash But you can do it, by using a module that resembles our ‘tempcdev.c’ demo (if you just modify its module-data appropriately)

In-class exercise #1 Download a fresh copy of our ‘tempcdev.c’ module and edit it, so that it will create the ‘/dev/stash’ device-file when you install it Then you can try using our ‘stash.c’ demo to send data from one task to another task by using the ‘echo’ and ‘cat’ commands

In-class exercise #2 Add a ‘get_info()’ function to this driver to create a pseudo-file (named ‘/proc/stash’) that will show the current contents of the ringbuffer (if any) and the current values for the ‘head’ and ‘tail’ buffer-indices Don’t forget: use ‘create_proc_info_entry()’ in your ‘init_module()’ function, and use ‘remove_proc_entry()’ during ‘cleanup’